1. Field of the Invention
This invention relates generally to electronic article surveillance systems and, more particularly, to electronic article surveillance systems of the type that detect a resonant marker or tag that is placed in a swept frequency electromagnetic field near the exit to a protected area. The system detects perturbations or tag signals that are generated when the frequency of the swept field passes through the resonant frequency of the tag to provide an alarm signal.
2. Description of the Prior Art
Swept frequency electronic article surveillance systems are known. One such system is described in U.S. Pat. No. 4,812,822. One of the problems that is encountered by electronic article surveillance systems, including the one described in the afore-mentioned U.S. Pat. No. 4,812,822, is that the signal produced by the marker or tag is generally quite small and the systems must work in noisy environments and be able to discriminate a valid tag signal from spurious radiations. Such spurious radiations may take the form of interfering carriers and resonances that have the same characteristics as a tag signal but are caused by building structures or other metallic structures in the vicinity that have resonance characteristics similar to those of a tag.
In order to provide the required discrimination between a tag signal and a spurious signal, the systems according to the prior art utilize relatively elaborate signal processing techniques including auto-correlation and various filtering techniques including synchronous integration, as described in the aforementioned mentioned U.S. Pat. No. 4,812,822, to discriminate between a valid tag signal and spurious signals or to filter out spurious signals. Other examples of attempts to eliminate spurious signals are disclosed in U.S. Pat. Nos. 4,117,466 and 4,168,496. U.S. Pat. No. 4,117,466 addresses the problem of filtering out an interfering carrier by detecting the beat frequency produced by the interfering carrier and the swept carrier of the system and inhibiting the alarm. The system disclosed in U.S. Pat. No. 4,168,496 addresses the problem of spurious signals produced by resonant structures in the area that generate a signal that looks like a tag signal. In the aforementioned mentioned system, the spurious tag-like signal is sampled and stored, and the stored signal is subsequently subtracted from the received signal to thereby cancel out the spurious signal from the received signal so that it is not detected as a valid tag signal. While the aforementioned systems do provide a way to distinguish between spurious and valid tag signals, they are relatively complex and different approaches must be taken to discriminate against different types of interfering signals, such as interfering carriers and resonances.